The present invention generally relates to transmission rate control systems, and more particularly to a transmission rate control system for an information processing system which exchanges data via a serial interface.
Recently, terminal equipment which is coupled to a personal computer or the like is controlled by data transmitted via a serial interface, so as to simplify the construction of the equipment and to simplify the control procedure.
A host unit which is coupled to such terminal equipment is provided with a number of serial interfaces corresponding to the number of terminal equipment coupled thereto. Hence, the host unit must simultaneously control the terminal equipment. On the other hand, in order to carry out the data processing within a short time, it is desirable that the transmission rate between the equipment be high.
There is a limit to the data processing speed within the host unit. For this reason, there is a possibility that data processing will become impossible if the transmission rate becomes too high.
For the sake of convenience, a case will be considered where the transmission rate is 10,000 bps and the start-stop transmission system is employed. If one byte of data is made up of a total of 10 bits, including 8 bits of unit data and 1 bit each of start and stop bits, it takes 1/1,000 second to transmit the data amounting to one byte.
When the transmission data is transferred continuously, the host unit must process the data within 1/1,000 second from the receipt thereof because the data will otherwise be over-written and erased by the next transmission data. If the data is erased, it no longer becomes possible to carry out normal data processing.
In addition, if there are 10 lines and 10 serial interfaces provided, the time in which the above data processing must take place becomes 1/10 of the above described 1/10,000 second.
Therefore, the transmission time between the host unit and the terminal equipment must be set depending on the performance of the host unit and the amount of data which is transmitted.
FIG. 1 shows an example of a conventional method of setting the transmission rate for each terminal equipment. A clock signal which is generated from a clock generator 48 of a terminal equipment 42 is supplied to a clock switching part 49 where the period of this clock signal is switched. The transmission rate of the terminal equipment 42 is determined by this clock signal output from the clock switching part 49. In the terminal equipment 42, the clock switching part 49 is set to 1,000 bps, and thus, the transmission rate is set to 1,000 bps.
Similarly, in terminal equipment 43 which is provided with a clock generator 50 and a clock switching part 51, the transmission rate is set to 5,000 bps. In terminal equipment 44 which is provided with a clock generator 52 and a clock switching part 53, the transmission rate is set to 10,000 bps. As may be seen from FIG. 1, the transmission rates of the terminal equipment 42, 43 and 44 can be set independently to arbitrary values.
Serial interfaces 45, 46 and 47 of a host unit 41 are provided in correspondence with the terminal equipments 42, 43 and 44. During the initial setting of the terminal equipments 42, 43 and 44, the serial interfaces 45, 46 and 47 respectively set the transmission rates with respect to the terminal equipment 42, 43 and 44.
According to this method, however, the transmission rates are fixed by the initial setting. For this reason, it is impossible to switch the transmission rates during operation of the terminal equipments 42, 43 and 44.
There are cases where the transmission data includes a large amount of data, such as image data. In such cases, it takes time to transmit the data from the terminal equipment if the transmission rate is low, and the operation speed of the terminal equipment deteriorates. Hence, it is necessary in such cases to increase the transmission rate depending on the amount of data, so that the transmission data can be transmitted collectively.
On the other hand, if the amount of the transmitted data is too large and the host unit cannot carry out the data processing, it is necessary to reduce the amount of transmitted data by reducing the transmission rate of the terminal equipment, so that the host unit can carry out normal data processing.
Therefore, even in the case where the data processing efficiency would deteriorate if the transmission rate of the terminal equipment is not changed during operation, the transmission rate cannot be changed when it is fixed by the initial setting. For this reason, there is a demand to realize a method of changing the transmission rate of the serial interface depending on the needs.
On the other hand, FIG. 2 shows an example of another conventional method of switching the transmission rate. According to this method, a command for changing the transmission rate is provided within a terminal equipment control signal which is transmitted to a terminal equipment 32 via a serial interface controller 33, a transmission line 36 and a serial interface controller 34. FIG. 3 is a time chart for explaining this method.
In FIG. 3, the data transmission is normally made at 1,000 bps. When switching the transmission rate from the normal 1,000 bps to 5,000 bps, a transmission rate change request command is transmitted from a host unit 31 to the terminal equipment 32. The terminal equipment 32 recognizes the transmission rate change request command within the transmission data, and changes in a clock switching part (transmission rate switching part) 35 the period of the clock signal which determines the transmission rate. The clock signal having the changed period is input to the serial interface controller 34, and the serial interface controller 34 determines the transmission rate with reference to this clock signal.
The data communication between the host unit 31 and the terminal equipment 32 is started at 5,000 bps after an answer related to the completion of the transmission rate change is transmitted from the terminal equipment 32.
When making the data transmission at 1,000 bps again, a transmission rate change request command is transmitted from the host unit 31 to the terminal equipment 32. In this case, the terminal equipment 32 changes the period of the clock signal which is generated from an oscillator 37, similarly to the above described case where the transmission rate is changed to 5,000 bps, and switches the transmission rate to 1,000 bps.
According to this method, the transmission rate can be set high when transmitting a large amount of data, such as when transmitting the image data. On the other hand, the transmission rate can be set to a relatively low value when making the normal data transmission.
However, this conventional method described with reference to FIGS. 2 and 3 suffer from the following problems.
When a power failure occurs at terminal equipment 32, the terminal equipment 32 is reset to the initial setting. If the transmission rate is set to 1,000 bps by the initial setting, for example, the transmission rate of the terminal equipment 32 is returned to 1,000 bps regardless of the transmission rate used at the time when the power failure occurred.
On the other hand, the host unit 31 recognizes that the transmission rate of the terminal equipment 32 is set to 5,000 bps at the time when the transmission rate change command is generated from the terminal equipment 32. The host unit 31 does not have any means for confirming the data transmission rate after the data transmission. For this reason, if a fault such as a power failure occurs at the terminal equipment 32 and the transmission rate is returned to the initial 1,000 bps after the recovery, the host unit 31 still recognizes the transmission rate of the terminal equipment 32 as being set to 5,000 bps which was used at the time when the fault occurred, and the host unit 31 carries out a process to exchange data with the terminal equipment 32 at 5,000 bps.
Accordingly, the transmission rate recognized by the host unit 31 becomes different from the transmission rate which is actually set at the terminal equipment 32. As a result, a synchronization error is generated when exchanging the data, and a correct data transmission can no longer be made between the host unit 31 and the terminal equipment 32.
If the supply of power to the terminal equipment is made from the host unit, the transmission rate of the terminal equipment will not return to the initial value even when a power failure occurs, and the above described problems will not occur. Similarly, the above described problems will not occur if the equipment is provided with the necessary control functions for making the recovery.
However, not all terminal equipment can receive the supply of power from the host unit. In addition, because the control procedure for making the recovery is extremely complex, the construction of the equipment becomes complex and the equipment becomes expensive. Therefore, it is desirable that the above described problems can be eliminated even if the terminal equipment does not receive the supply of power from the host unit.